1. Field of Invention
This invention relates generally to a fuel injection system and method to control emissions from a Diesel engine, and more particularly to a method that controls smoke (soot) and/or nitrogen oxide (NOx) emissions by co-injecting a mixture of oxygenated fuel and water during steady state and engine transient conditions.
2. Description of Related Art
Diesel engine and truck manufacturers are under increasing pressure to reduce smoke and NOx levels in the exhaust. Current generation engines rely on high-pressure injection systems, electronic controls, and exhaust gas recirculation (EGR) to mitigate smoke and NOx emissions while attempting to maintain high fuel efficiency. However, the most significant gains from these technologies have probably already been achieved. Water injection into the cylinder is not widely used in current engines, but is well known to be an effective means of reducing NOx and smoke emissions.
Improvements in fuel formulations are likely to yield significant emissions benefits in the future, as research in this area is still relatively new. Recent tests have shown that alternative fuels and mixtures thereof can significantly reduce smoke and NOx simultaneously, while preserving the high efficiency and high power density of the Diesel-cycle engine.
In-cylinder water injection into the combustion chamber of a Diesel engine is one well-recognized method for reducing NOx and smoke emissions. Water injection either as a fuel/water emulsion, or as a separate fluid, can reduce NOx and smoke emissions. The most common approaches have involved the use of emulsified fuels, stored in a tank or mixed in-line, or water injection through a nozzle separate from the fuel injector. When using emulsified fuels, the water content cannot be altered quickly enough for transient control. When injected separately, the water injection amount can be varied quickly, but this method does not reduce smoke emissions appreciably because water acts primarily as a diluent. In addition, water does not contribute appreciably to the energy release.
Using oxygenated fuel is another recognized approach for reducing particulate smoke emissions from Diesel engines. An oxygenated fuel is a fuel in which oxygen atoms are chemically bound within the fuel structure. The oxygen bond in the oxygenated fuel is energetic and provides a chemical energy that results in no loss of efficiency during combustion. However, oxygenated fuels have only a minor effect on NOx emissions.
An exhaust gas recirculation (EGR) system is yet another recognized alternative for reducing NOx emissions levels from internal combustion engines. The exhaust gas recirculation system recirculates exhaust gases through the intake manifold to be mixed with fresh intake air and burned again. The exhaust gas recirculation system dilutes the air and fuel mixture. Consequently, the peak combustion temperature is lowered and NOx production is decreased. EGR has a detrimental effect on soot and smoke emissions.
The present invention is directed to overcoming the problems set forth above. This invention relates to a method for simultaneously reducing NOx and smoke emissions from a compression ignition (e.g., a Diesel cycle) engine. The invention provides for the combined use of water injection and an oxygenated fuel. This is particularly suitable for engines that use exhaust gas recirculation (EGR), but is not limited to this category of engine.
According to one aspect of the invention, oxygenated fuel and water are co-injected into the combustion chamber of a compression ignition engine. This results in a synergistic improvement in both the NOx emissions and the particulate (soot or smoke) emissions, as compared to the use of oxygenated fuel alone, or co-injection of water and regular fuel. The co-injection can be conducted with or without exhaust gas recirculation (EGR).
The oxygenated fuel and water can be injected through a single injector, or the oxygenated fuel can be injected through a first injector, while the water is injected through a second injector.
The water can be introduced as water alone, or as part of a fuel emulsion. When a fuel emulsion is used, the fuel emulsion is the main fuel supply, whereas the oxygenated fuel is a secondary fuel supply.